Spontaneous magnetic reconnection: Collisionless reconnection and its potential astrophysical relevance


A concise review is given on the relevance of reconnection in the astrophysical context. Emphasis is put on recent developments in reconnection theory provided by collisionless numerical simulations in 2d and 3d. It is stressed that magnetic reconnection is a \emph{universal} process of particular importance under collisionless conditions when classical collisional and anomalous dissipation are both irrelevant. While collisional (resistive) reconnection is a diffusive slow process, collisionless reconnection is spontaneous (on any astrophysical time scale it is explosive), setting on when electric current widths become comparable to the leptonic (electrons/positrons) inertial length in the so-called lepton ``diffusion region'' where leptons de-magnetise, allowing the magnetic field to annihilate with its oppositely directed partner. Spontaneous reconnection breaks the original magnetic symmetry, violently releasing the stored free energy of the electric current, and causing plasma heating and particle acceleration. Ultimately the released energy is provided by mechanical motion of either the two colliding magnetised plasmas that generate the current sheet, or the internal turbulence cascading down to lepton-scale current filaments. Spontaneous reconnection in extended current sheets separating two colliding plasmas results in generation of many reconnection sites (tearing modes) distributed over the current surface, each consisting of lepton exhausts and jets separated by plasmoids. Volume filling factors of reconnection sites are estimated as large as $<10^{-5}$ per current sheet. Lepton currents inside exhausts bifurcate and break off into many small scale current filaments and magnetic flux ropes exhibiting turbulent magnetic power spectra of power law shape $W_b\propto k^{-2}$. Spontaneous reconnection is a generator of small-scale turbulence. Imposed external turbulence tends increasing the reconnection rate. Reconnecting ultra-relativistic current sheets decay into large numbers of magnetic flux ropes composed of chains of plasmoids and lepton exhausts forming a highly structured current surface. Including synchrotron radiation losses favours tearing mode reconnection over the drift-kink deformation of the current sheet. Lepton acceleration takes place by the reconnection electric field in multiple encounters with the exhausts and plasmoids and is a Fermi-like process. It results in power law tails on the lepton energy distribution. This effect becomes strong in ultra-relativistic reconnection yielding extremely hard power law energy spectra approaching $F(\gamma)\propto \gamma^{-1}$ far exceeding the synchrotron radiation limit- Relativistic reconnection thus becomes a most probable generator of current and magnetic turbulence and mechanism of high energy radiation. It is also identified as the ultimate dissipation mechanism of the mechanical energy in collisionless magnetohydrodynamic turbulent cascades via lepton scale turbulent current filaments. In this case the volume filling factor is large. Turbulence thus causes strong plasma heating of the entire turbulent volume and violent acceleration leading to nonthermal radiation high-energy particle populations.

Further Information
  author = {Treumann, R. A. and Baumjohann, W.},
  doi = {10-1007/s00159-015-0087-1},
  journal = {Astron. Astrophys. Rev.},
  language = {en},
  note = {19 Figures},
  number = {4},
  pages = {1-91},
  title = {Spontaneous magnetic reconnection: Collisionless reconnection and its potential astrophysical relevance},
  url = {http://dx.doi.org/10.1007/s00159-015-0087-1},
  volume = {23},
  year = {2015},
%O Journal Article
%A Treumann, R. A.
%A Baumjohann, W.
%R 10-1007/s00159-015-0087-1
%J Astron. Astrophys. Rev.
%G en
%O 19 Figures
%N 4
%P 1-91
%T Spontaneous magnetic reconnection: Collisionless reconnection and its potential astrophysical relevance
%U http://dx.doi.org/10.1007/s00159-015-0087-1
%V 23
%D 2015